140 



HISTORICAL PALEONTOLOGY. 



first coiled up, crosier-fashion, like the young fronds of ferns, 

 whilst the old branches carry numerous spore-cases. The 

 stems and branches seem to have attained a height of two or 

 three feet ; and they sprang from prostrate " root-stocks " or 



creeping stems. Upon the whole, 

 Principal Dawson is disposed to 

 regard Psilophyton as a " general- 

 ized type" of plants intermediate 

 between the Ferns and the Club- 

 mosses. Lastly, the Devonian de- 

 posits have yielded the remains of 

 the first actual trees with which we- 

 are as yet acquainted. About the 

 nature of some of these (Ormoxy- 

 lon and Dadoxylon} no doubt can 

 be entertained, "since their trunks 

 not only show the concentric rings 

 of growth characteristic of exog- 

 enous trees in general, but their 

 woody tissue exhibits under the 

 microscope the " discs " which are 

 characteristic of the wood of the 

 Pines and Firs (see fig. 2). The 

 singular genus Prototaxites, how- 

 ever, which occurs in an older por- 

 tion of the Devonian series than 

 the above, is not in an absolutely 

 unchallenged position. By Prin- 

 cipal Dawson it is regarded as the 

 trunk of an ancient Conifer the 

 most ancient known ; but Mr. 

 Carruthers regards it as more 

 probably the stem of a gigantic 

 sea-weed. The trunks of Proto- 

 taxites (fig. 78, A) vary from one 

 to three feet in diameter, and 

 exhibit concentric rings of growth ; 

 but its woody fibres have not 

 hitherto been clearly demonstrated 

 to possess discs. Before leaving 



the Devonian vegetation, it may be mentioned that the hornstone 

 or chert so abundant in the Corniferous limestone of North 

 America has been shown to contain the remains of various 



Fig. 11. Restoration of Psilo- 

 phyton princeps Devonian, Can- 

 ada. (After Dawson.) 



